Present methods for suspended solids removal include filtration, sedimentation and flotation. The method closest to the present invention is gas flotation wherein the waste water stream is conditioned with a coagulant or coagulants, and a polyelectrolyte polymer, then injected with a gas such as air and retained for a period of time in a vessel. Retention allows a blanket of solids to form on top of the retained waste water. The blanket is skimmed off and further processed. This method and the vessel with its skimmers, piping, and controls are well known to those skilled in the art as are the coagulants and polymers used. It is known to add metal salts, organic coagulants, or bentonite and a polymer in certain combinations and quantities to a waste water stream wherein the gas flotation method is used to clarify the waste water stream. Such known methods, typically, result in formation of a blanket of solids and water to be removed which contains about 5 to 25% solids.
The removed solids include, of course, those contained in the waste water stream, as well as, those added to induce the solids to float and form a blanket. Since the blanket from a waste water stream resulting from food processing must be disposed of or further processed, it is more economical and more environmentally acceptable to render the blanket (wherein the produce is cooked and water removed) and return the solids to the animal food chain as an ingredient in cattle, chicken or other animal foods. The known prior art methods add oxidizing chemicals and/or other solids to the waste which can reduce the value of the recovered solids.
Prior art methods employing gas flotation are capable of floating particles from a colloidal suspension by destabilizing the electrical charge of the particles, by the use of a coagulant and by adding a polymer with an opposite charge. Thus, the compounds added are either anionic, cationic or nonionic. The result, electrically, of the additions should be to have particles of such a charge that the gas bubbles will be attached to the solids and float the agglomerates to the surface. Other problems in the prior art led to the present invention, to wit: metal salts used as coagulants are not desirable in material to be returned to the animal food chain and metal salts are also undesirable because of their tendency to coat the cooker, thereby reducing efficiency and creating a constant need for cleaning which is time-consuming and costly. Also, where bentonite is used as a coagulant, a cationic polymer is commonly needed to cause flocculation for acceptable flotation. Cationic polymers have been found to have toxic qualities and thus are not appropriate as an additive for animal feed.
Many anionic polymers are typically safe for animal consumption. Several patents are cited herebelow that disclose the use of an anionic polymer with bentonite to promote the flocculation of solids in waste water. Japanese Patents Nos. 53-45051 and 53-93655 issued to Toshin suggest that an anionic or cationic polymer may be used with bentonite as a coagulant. Japanese Patent No. 54-16845 issued to Ebara also discloses the use of either an anionic or nonionic polymer with bentonite to clarify waste water. U.S. Pat. No. 3,128,249 and Australian Patent No. 215,787, both issued to Pye, disclose a method for clarifying waste water wherein an acrylamide polymer is mixed with bentonite to flocculate suspended solids.
It should be noted that the above-mentioned prior art does not contemplate using the recovered solids as animal feed. Thus, the prior art does not contemplate the use of selected coagulants specifically for their characteristic as a safe, non-toxic food additive nor does the prior art contemplate maximizing the concentration of recovered solids to facilitate a profitable processing of these solids into animal feed. Reducing the turbidity of the waste water to an acceptable level is the primary goal of the prior art. It has been the experience of the applicants and the industry that using an anionic polymer with bentonite to clarify waste water will reduce the turbidity of the water; however, the recovery of waste solids in relation to the amount of introduced coagulant has proven to be adequate at best. The prior art, though acceptable in the industry for which it was developed, may not provide sufficient or consistently reliable recoveries of solids to sustain an economically viable processing of the recovered solids into animal feed. As will be shown hereafter, the waste water clarification methods set forth in the prior art have been improved by the present invention to maximize the recovery of solids from waste water.